We can’t see this possible 9th planet, but we feel its presence

January 22, 2016 at 6:35 PM EST

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Most of us grew up thinking there were nine planets in the solar system, but that changed when Pluto got downgraded in 2006. Now there's news that there might be a ninth planet after all. Researchers have found evidence of a planet with a mass 10 times that of Earth. Jeffrey Brown talks to Mike Brown of the California Institute of Technology.

HARI SREENIVASAN: Next: a big finding and discovery in space that has astronomers and plenty of other people excited.

Jeffrey Brown has our conversation.

JEFFREY BROWN: Nine planets in the solar system, right? That’s what most of us grew up thinking. Well, that ended in 2006, when Pluto was downgraded to a dwarf planet.

But now comes news that there might be a ninth planet after all. Researchers at the California Institute of Technology found evidence of a planet with a mass 10 times that of Earth.

One of the Caltech astronomers, Mike Brown, joins us now. And I will add that he is also known as the chief culprit in lowering Pluto’s status. His memoir is titled “How I Killed Pluto and Why It Had It Coming.”

So, sort of making amends here I guess, Mike Brown?

MIKE BROWN, California Institute of Technology: You know, it’s — I don’t think of it that way. I think of it as, this is something I have been working at for 20 years, and Pluto was just collateral damage along the way.

JEFFREY BROWN: All right, so let’s be as clear as we can about what you have done here. You have not seen this new planet, right? This is something you surmise. Explain to us.

MIKE BROWN: Yes, that’s absolutely right, and it’s important to know that we — no one has actually seen this planet yet.

What we have done is felt it, or, more precisely, we have seen its gravitational effect on the most distant things in the solar system. And from those gravitational effects, we can tell that it must be out there. And this is the same way that Neptune, for example, was discovered, by its gravitational effects on Uranus. So, there’s a long history of this sort of astronomy.

JEFFREY BROWN: You’re seeing gravitational effects on several — I gather, six small bodies out there?

MIKE BROWN: There is actually quite a big collection.

There are six that are doing one thing. There are five more that are doing something else. And then there’s another eight doing something. When you put them all together, it’s a pretty big population that are going in directions and moving in ways that they shouldn’t be doing unless there is something organizing the whole pattern.

JEFFREY BROWN: So, this is — I don’t know if this is a silly question, but this is what confused me. If you can see — or can see those bodies, why can you not see the planet, if you know those bodies are there for sure?

MIKE BROWN: Yes, so that’s a really good question.

The planet — the effect of the planet can be felt by probably many thousands of bodies that are out there. We have only found the first score of them. But there must be many thousands out there. So it’s a lot easier to look in a random direction in space and see something that has been affected by this planet than to immediately go and find it.

So, right now, we know the orbital path of the planet through the sky, but we don’t know where in the orbital path it is, so we’re setting out on a grand search to try to find it.

JEFFREY BROWN: Well, so what — if it is a planet, what do — what else you know about it in terms of size, mass, why it’s so far away from the other known planets?

MIKE BROWN: It’s actually — having not seen it, we actually know quite a good bit about it, or we have inferred quite a good bit about it.

So we know that it’s about 10 times the mass of the Earth. It has to be that big to have had the gravitational influence over such a long distance that it has. And at 10 times the mass of the Earth, that makes it a little bit smaller than Neptune.

So, we think it’s probably a miniature Neptune. It’s probably a gas giant like Neptune. It has no solid surface, just a gas surface. And it has this crazy elliptical orbit. It takes 10,000 to 20,000 years to go around the sun. And the closest it ever gets to the sun is about 20 billion miles, which, to put it in context, is — it’s something like five times further than the distance that Pluto is right.

But on that 10,000-year journey around the sun, it goes out to 100 billion miles, which is about 25 times the distance of Pluto. So, it’s really quite a long ways out there and on quite an extended orbit.

JEFFREY BROWN: This is not, of course, the first time that astronomers have suspected the presence of a phantom planet at the outer edges of the solar system, right? There is reason to be more confident this time?

MIKE BROWN: Yes, so, actually, the idea that there is another planet out beyond Neptune or beyond Pluto, when Pluto was a planet, has a long and, I would say, fairly sordid history over the past hundred years, really.

JEFFREY BROWN: Yes.

MIKE BROWN: Every time astronomers would see something that they didn’t quite understand in the outer part of the solar system, the immediate thing that everybody jumps to is must be a planet.

So, you can go back really over the last century and read all these different accounts of why there must be a planet. Every single one of them has turned out to be wrong. Usually, it’s that the data they were looking at were incorrect. Sometimes, they were misinterpreting data. But they have always been wrong.

So, we’re saying, yes, for 100 years, everybody’s been wrong, but we’re, of course, right.

(LAUGHTER)

JEFFREY BROWN: Of course. I will go with you for now.

But, also, this made me think about, and for the layman, how this kind of science is done. Right? This is always a mix of theory and observing, that they both work hand in hand?

MIKE BROWN: Yes, this has actually been — that mix is personified in the two of us have been working on this problem.

So, it’s me. I’m primarily an observational astronomer. I look at the sky and try to figure out what’s going on. My colleague, who I think did the heavy lifting here, is Konstantin Batygin. He is a theorist. He understands the gravitational dynamics in gory detail.

And he and I are four doors down from each other at Caltech. And we would be running back and forth across the hallways for the past two years, really. I would bring in some more observational constraints. He would think about it, put it in the computer, write the equations. We would talk to each other. We would argue.

It’s — these two different viewpoints, I think, were critical to really making this realization that there has to be this planet out there.

JEFFREY BROWN: Well, so what happens next? How will scientists verify its existence? Do you ultimately have to see it?

MIKE BROWN: You have to see it.

Right now, any good scientist is going to be skeptical, because it’s a pretty big claim. And without the final evidence that it’s real, there is always that chance that it’s not. So, everybody should be skeptical. But I think it’s time to mount this search.

I mean, we like to think of it as, we have provided the treasure map of where this ninth planet is, and we have done the starting gun, and now it’s a race to actually point your telescope at the right spot in the sky and make that discovery of planet nine.

JEFFREY BROWN: All right, Mike Brown of Caltech, thanks so much, and good luck.

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